The prior art technology will now be described with reference to the case of cleaning air in cleanrooms at semiconductor manufacturing shops.
In cleanrooms, fine particles (particulate matter) and gaseous substances such as extremely low concentrations of non-methane hydrocarbons (HCs) in air originating typically from automotive emission gases are of importance as contaminants. In particular, HCs, if present at extremely low concentrations in ordinary air (both inside and outside rooms), will cause contamination as gaseous deleterious components and, hence, need be removed. Various solvents (e.g. alcohols and ketones) that accompany operations performed in cleanrooms are also of importance as contaminants.
Stated more specifically, if the above-mentioned contaminants (fine particulate and gaseous contaminants) deposit on the surfaces of substrates as wafers, semi-finished products and finished products, the substrate surfaces are prone to be damaged and this can be a cause of a lower production rate (yield) of semiconductor products; hence, those contaminants have to be removed. Both fine particles and gaseous substances will increase the angle of contact on substrate surfaces and, particularly in cleanrooms, HCs have been found to have a great tendency to increase the angle of contact. The term “angle of contact” as used herein refers to the angle of contact due to wetting by water and indicates the degree of contamination on substrate surfaces. Stated more specifically, when a hydrophobic (oily) substance deposits on a substrate surface, said surface repels water to become less likely to be wetted. Then, the angle of contact between the substrate surface and the drop of water will increase. Therefore, a large angle of contact means a high degree of contamination and, conversely, a small angle of contact means a low degree of contamination.
Conventional methods of cleaning air in cleanrooms or apparatus therefor are roughly classified in two categories:                (1) a mechanical filtering method (e.g. a HEPA filter); and        (2) a filtering approach (e.g. HESA filter) that entraps fine particles electrostatically by means of a high-voltage charged or conductive filter. Either of these methods aims at removing fine particles and is ineffective for removing gaseous contaminants such as non-methane hydrocarbons (HCs) that will increase the angle of contact.        
On the other hand, HCs which are gaseous contaminants are known to be removable by such methods as combustion decomposition and O3 decomposition. However, these methods are ineffective for removing extremely low concentrations of HCs present in air to be introduced into cleanrooms.
Gaseous deleterious components other than HCs include SOx, NOx, HCl, NH3, etc. and these are known to be removable by relying either upon neutralization reactions using appropriate alkaline or acidic substances or upon oxidation reactions. However, these methods are also less effective if the concentrations of the components of interest are extremely low as in the case where they are present in air to be introduced into cleanrooms.
The present inventors already proposed methods and apparatus for preventing the contamination of substrate surfaces by using adsorbents or absorbers with a view to preventing the increase in the above-defined angle of contact (Japanese Patent Application Nos. 91/341802 and 92/180538). These methods and apparatus are effective in certain areas of application but further improvements are necessary in order to increase their practical feasibility.
Stated more specifically, particulate matter and gaseous deleterious components that increase the angle of contact must be removed in order to improve the production rate of semiconductor products. Hence, the object of the present invention is to provide a method and an apparatus for preparing clean gases in which the concentrations of fine particles and gaseous deleterious components that will increase the angle of contact on substrate surfaces are reasonably low.